This invention relates to a DC level control circuit and, more particularly, to a clamping or DC restoration circuit which finds ready application in television signal receiving apparatus.
Clamping circuits are generally used to fix or "clamp" a given signal to a preselected DC level. By the use of such clamping circuits, the information contained in the varying portion of such given signal is not affected by the DC reference level is changed. In a typical application of a clamping circuit, the DC component of a television signal is restored after the television signal is subjected to various stages of processing in a television receiver. Since the DC level of a video signal is used to represent the brightness of a reproduced video picture and is additionally used as a reference level for detecting the synchronizing signal pulses included therein, it often is necessary to re-insert into the video signal the DC level which is removed by various video amplifiers. This, of course, corrects for a loss in the brightness level of the reproduced picture and, additionally, enables the synchronizing signal pulses to be properly detected.
In a conventional clamping circuit of simplified construction, a capacitor is connected to a diode rectifier which is poled in a direction whereby the capacitor is permitted to charge to a peak value of the video input signal. Generally, the synchronizing signal pulse of a video signal is the negative peak value of the video signal. Accordingly, the diode may be poled in a direction to permit the capacitor to charge to this negative peak value. During information periods between the synchronizing signal pulses, the output signal obtained across the diode is of the same shape as the input video signal, but now has its DC level displaced in accordance with the negative peak value stored by the capactior. Thus, the synchronizing signal peaks are fixed or "clamped" to a predetermined level determined by the bias which might be applied to the rectifier diode.
The aforedescribed conventional clamping circuit is not provided with any signal feedback. Thus, the stability of this circuit to clamp or restore the DC level of the video signal is less than perfect. Moreover, the conventional circuit is deleteriously influenced by variations in temperature, resulting in non-uniform DC clamping levels. Furthermore, since capacitors must be connected in series in the video signal path, it is often difficult to design the conventional clamping circuit in integrated circuit configuration.